1. Field
The subject matter disclosed herein relates to determining a location of a position based upon signals received from geo-location satellites.
2. Information
A satellite positioning system (SPS) typically comprises a system of earth orbiting satellites enabling entities to determine their location on the earth based, at least in part, on signals received from the satellites. Each such SPS satellite typically transmits a signal marked with a repeating pseudo-random noise (PN) code of 1,023 chips distinguishing the satellite from other SPS satellites where the 1,023 chips repeat every millisecond. The signal is also typically modulated with data bits, where each data bit has a 20 ms duration in the modulated signal.
FIG. 1 illustrates a typical application of a geo-location system, whereby a subscriber station 100 in a wireless communications system receives transmissions from satellites 102a, 102b, 102c, 102d in the line of sight to subscriber station 100, and derives time measurements from four or more of the transmissions. Subscriber station 100 provides the measurements to position determination entity (PDE) 104, which determines the position of the station from the measurements. Alternatively, the subscriber station 100 may determine its own position from this information.
Subscriber station 100 may search for a transmission from a particular satellite by correlating the PN code for the satellite with a received signal. The received signal typically comprises a composite of transmissions from one or more satellites within a line of sight to a receiver at station 100 in the presence of noise. A correlation may be performed over a range of code phase hypotheses known as the code phase search window WCP, and over a range of Doppler frequency hypotheses known as the Doppler search window WDOPP. Such code phase hypotheses are typically represented as a range of PN code shifts while such Doppler frequency hypotheses are typically represented as Doppler frequency bins.
A correlation is typically performed over an integration time “I” which may be expressed as the product of Nc and M, where Nc, is the coherent integration time, and M is number of coherent integrations which are non-coherently combined. For a particular PN code, correlation values are typically associated with corresponding PN code shifts and Doppler bins to define a two-dimensional correlation function. Peaks of the correlation function are located and compared to a predetermined noise threshold. The threshold is typically selected so that the false alarm probability, the probability of falsely detecting a satellite transmission, is at or below a predetermined value. A time measurement for the satellite is typically derived from a location of an earliest non-side lobe peak along the code phase dimension which equals or exceeds the threshold. A Doppler measurement for the subscriber station may be derived from the location of the earliest non-side lobe peak along the Doppler frequency dimension which equals or exceeds the threshold.
Current subscriber station architectures place significant constraints on the process of searching for location determination signals. In a shared RF architecture, for example, core RF circuitry in the subscriber station is typically shared between a location determination receive path, and voice/data communication transmit and receive paths. Accordingly, employing such a shared RF architecture in an SPS function may diminish an ability of such a shared architecture to perform a voice/data communication function or other function sharing common resources. Accordingly, there is a desire to reduce use of such common resources for determining locations of position.